1900s
- 1902 - Dominion Observatory, Ottawa, Canada established
- 1904 - Observatories of the Carnegie Institution of Washington founded
- 1907 - F.C. Brown and Joel Stebbins develop a selenium cell photometer at the University of Illinois Observatory.
Read more about this topic: Timeline Of Telescopes, Observatories, And Observing Technology, Common Era (A.D.)